Cross table lateral radiography for measurement of acetabular cup version

Introduction

Anteversion of the acetabular component is an important factor for long-term results of total hip arthroplasty (THA). It is generally accepted that the acetabular component should be placed with an anteversion of 15°±10°, and inaccurate placement can cause impingement, dislocation and accelerated wear (1-5). Computed tomography (CT) is considered the most accurate method for measurement of cup anteversion (6), but in clinical practice conventional radiology is most commonly used because it is easier to implement, has low emissions, and low cost (7). While the inclination of the acetabulum easily can be measured from standardized anteroposterior (AP) views, the calculation of anteversion is more difficult. Therefore, acetabular anteversion has been measured from cross-table lateral radiographs (8). However, the accuracy of conventional radiography to measure the acetabular component anteversion after THA is controversial (6), and in this study we sought to determine the accuracy of cross-table lateral radiography as compared to CT for measuring anteversion of the acetabular component.

Methods

The study was approved by the Regional Ethics Committee of South East Norway and performed in accordance with the ethical standards of the Declaration of Helsinki. After informed consent to participate, 71 women and 46 men (117 hips) aged 48 to 81 (mean 66) years were prospectively enrolled in the study. They all underwent cementless THA and consecutively were recruited into the study. We used a porous coated hemispherical press fit cup, and we aimed to obtain acetabular anteversion of 10° to 30° using a cup positioner with an alignment connector and frame provided with the implant. This is designed to obtain 20° of anteversion if the alignment rod is in line with the longitudinal axis of the patient.

At 3 months postoperatively, and after informed consent, the acetabular component version was measured by cross table lateral radiography. It was carried out with the patient in a supine and neutral position. The contralateral was flexed 45 degrees and placed on a small stand to keep the position. The direction of the radiation beam was parallel to the examination table with 45° to the long axis of the body, and the X-ray film was perpendicular to the examination table. The acetabular component version was determined as the angle in degrees between a line drawn along the angle of the rim of the cup and a line perpendicular to the horizontal plane (Figure 1).This was compared with the version measured by CT (General Electric LightSpeed Pro 16 Milwaukee, Wi, USA). Single scans, 10 mm of thickness, were made through the centre of the femoral head, and the angle between a line connecting the lateral anterior and posterior margins of the acetabular component and the coronal plane defined as the plane perpendicular to a line connecting two identical points on either side of the pelvis was measured (Figure 2). The same specialist in radiology made all assessments.

Figure 1 Acetabular anteversion of 11.4 degrees measured by cross table lateral radiography.

Figure 2 Acetabular anteversion of 12.1 degrees measured by computed tomography.

X-ray and CT measurements were described with mean, standard deviation (SD) and 95% confidence interval (CI). The agreement of measurements from X-ray and CT were analyzed by calculating the intraclass correlation coefficient (ICC). It is a measure of the proportion of variance that is attributed to the individual patients when they are all rated by the same rater. An ICC of 1 indicates a perfect agreement. A Bland-Altman plot with the mean of differences between measurements from X-ray and CT and the limits of 95% agreement (mean ± 1.96SD) was also conducted.

Results

The X-ray (mean 13.9, SD 10.1, 95% CI: 12.1–15.8) and CT (mean 17.8, SD 12.6, 95% CI: 15.4–20.1) measurements were obtained as described. The ICC was 0.789 (95% CI: 0.587–0.880). Mean difference (X-ray − CT) was −3.8 with −16.8 and 9.2 as 95% limits of agreement (Bland Altman plot Figure 3). It was a tendency to reversed difference between X-ray and CT measurements with increased average measurement, i.e., higher X-ray measurements compared with CT for lower average measurements and lower X-ray measurements compared with CT for higher average measurements. This was statistically assessed with a regression analysis of the data in the Bland Altman plot and a significant slope coefficient of −0.242 (95% CI: −0.345 to 0.139), P<0.001 was found.

Figure 3 Bland Altman plot: Bland-Altman plot for X-ray and CT measurements with mean difference of −3.8 (95% limits of agreement −16.8 to 9.2). CT, computed tomography.

Discussion

In this study we measured the acetabular cup anteversion on a lateral view radiograph and compared with CT scans. The results suggest that lateral view radiographs provide values that are on average four degrees lower than on CT scans, and that this bias may increase with higher values for acetabular component anteversion.

Different techniques for measurement of cup anteversion from plain AP views have been described, but with different conclusions concerning validity (9). There are various reasons for this. Anteversion has been defined in relation to different planes and landmarks, the tecniques require difficult trigonometric calculations and divergence of the X-ray beam as well as uncontrolled rotation of the pelvis are all factors that may affect the measurements (10). In a study by Nho et al. (11) radiographic anteversion calculated from plain AP radiographs was compared with anteversion measured on the CT, and it was concluded that the methods of Lewinnek et al. (1), Hassan et al. (12), Liaw et al. (13) and Woo et al. (14) were accurate, whereas the methods of Widmer (15) and Ackland et al. (16) were not.

The accuracy of cup anteversion on lateral view radiographs depends on the patient positioning whereas properly performed CT measurements are more independent of patient positioning (6). In our study the anteversion was measured in relation to the horizontal plane, and we assumed that the patient positioning was parallel to this plane using a standardized positioning protocol. However, a standardized pelvic tilting and rotation during imaging may be difficult, and a tilted pelvis changes the radiographic projection and distorts the measurement. Computed tomography decreases the effects of positioning of pelvic tilt, and we used CT as reference standard. But it should be emphasized that there is no gold standard for validation of radiographic or CT-based values in vivo. Our calculations were performed by one reviewer, but Ghelman et al. (6) demonstrated a strong intraobserver and interobserver reliability in the measurement of both conventional radiographs and CT scans for the assessment of acetabular version. We therefore assume that the use of a single reviewer should not adversely affect our observations.

While plain AP pelvic radiographs easily can be obtained, their accurate interpretation is subject to error (9,17,18). Both the direction of the central beam and tilt of the pelvis could influence the calculation of anteversion and cause significant errors (10). Centering of the X-ray beam over the hip versus the pubic symphysis, could theoretically reduce the sources of error with lateral view radiographs. However, Nishino et al. (19) compared acetabular component anteversion measured on lateral view and AP radiographs, and they found a difference of 2.8°±4.1° and −0.57°±3.1°, respectively as compared to CT scans. Nunley et al. (20) found a strong correlation between anteversion determined from lateral view radiographs and CT scans. Acetabular anteversion averaged 26.1° on lateral view imaging and 28.8° on CT scans. However, variation exceeded 10° for 20% of the patients. Ghelman et al. (6) found that measurements of anteversion on lateral view radiographs correlated (ICC =0.69) with those on CT (ICC =0.69). But the radiographs averaged 8.7° more anteversion than the CT measurements. Our results are on average between these observations. We found that lateral view measurements was on average 3.8 degrees lower than CT measurements with a distribution of measurements of ±13 degrees for 95% of the cases. This indicate that lateral view radiographs provides acceptable assessment for clinical use, but this imaging has limited use for precise analysis of acetabular component anteversion in THA.

Acknowledgements

We thank Are Hugo Pripp for statistical advices.

Footnote

Conflicts of Interest: The authors have no conflict of interest to declare.

Ethical Statement: This study was approved by the institutional ethic review board and informed consent was obtained from all patients.

References

1. Lewinnek GE, Lewis JL, Tarr R, et al. Dislocations after total hip-replacement arthroplasties. J Bone Joint Surg Am 1978;60:217-20. [PubMed]
2. Kennedy JG, Rogers WB, Soffe KE, et al. Effect of acetabular component orientation on recurrent dislocation, pelvic osteolysis, polyethylene wear, and component migration. J Arthroplasty 1998;13:530-4. [Crossref] [PubMed]
3. Jolles BM, Zangger P, Leyvraz PF. Factors predisposing to dislocation after primary total hip arthroplasty: a multivariate analysis. J Arthroplasty 2002;17:282-8. [Crossref] [PubMed]
4. Barrack RL. Dislocation after total hip arthroplasty: implant design and orientation. J Am Acad Orthop Surg 2003;11:89-99. [Crossref] [PubMed]
5. Wan Z, Boutary M, Dorr LD. The influence of acetabular component position on wear in total hip arthroplasty. J Arthroplasty 2008;23:51-6. [Crossref] [PubMed]
6. Ghelman B, Kepler CK, Lyman S, et al. CT outperforms radiography for determination of acetabular cup version after THA. Clin Orthop Relat Res 2009;467:2362-70. [Crossref] [PubMed]
7. Lu M, Zhou YX, Du H, et al. Reliability and validity of measuring acetabular component orientation by plain anteroposterior radiographs. Clin Orthop Relat Res 2013;471:2987-94. [Crossref] [PubMed]
8. Yao L, Yao J, Gold RH. Measurement of acetabular version on the axiolateral radiograph. Clin Orthop Relat Res 1995.106-11. [PubMed]
9. Marx A, von Knoch M, Pförtner J, et al. Misinterpretation of cup anteversion in total hip arthroplasty using planar radiography. Arch Orthop Trauma Surg 2006;126:487-92. [Crossref] [PubMed]
10. Goergen TG, Resnick D. Evaluation of acetabular anteversion following total hip arthroplasty: necessity of proper centring. Br J Radiol 1975;48:259-60. [Crossref] [PubMed]
11. Nho JH, Lee YK, Kim HJ, et al. Reliability and validity of measuring version of the acetabular component. J Bone Joint Surg Br 2012;94:32-6. [Crossref] [PubMed]
12. Hassan DM, Johnston GH, Dust WN, et al. Radiographic calculation of anteversion in acetabular prostheses. J Arthroplasty 1995;10:369-72. [Crossref] [PubMed]
13. Liaw CK, Hou SM, Yang RS, et al. A new tool for measuring cup orientation in total hip arthroplasties from plain radiographs. Clin Orthop Relat Res 2006.134-9. [Crossref] [PubMed]
14. Woo RY, Morrey BF. Dislocations after total hip arthroplasty. J Bone Joint Surg Am 1982;64:1295-306. [PubMed]
15. Widmer KH. A simplified method to determine acetabular cup anteversion from plain radiographs. J Arthroplasty 2004;19:387-90. [Crossref] [PubMed]
16. Ackland MK, Bourne WB, Uhthoff HK. Anteversion of the acetabular cup. Measurement of angle after total hip replacement. J Bone Joint Surg Br 1986;68:409-13. [PubMed]
17. Kalteis T, Handel M, Herold T, et al. Position of the acetabular cup -- accuracy of radiographic calculation compared to CT-based measurement. Eur J Radiol 2006;58:294-300. [Crossref] [PubMed]
18. Arai N, Nakamura S, Matsushita T, et al. Minimal radiation dose computed tomography for measurement of cup orientation in total hip arthroplasty. J Arthroplasty 2010;25:263-7. [Crossref] [PubMed]
19. Nishino H, Nakamura S, Arai N, et al. Accuracy and precision of version angle measurements of the acetabular component after total hip arthroplasty. J Arthroplasty 2013;28:1644-7. [Crossref] [PubMed]
20. Nunley RM, Keeney JA, Zhu J, et al. The reliability and variation of acetabular component anteversion measurements from cross-table lateral radiographs. J Arthroplasty 2011;26:84-7. [Crossref] [PubMed]